Preheat oven to 375°F. With a sharp knife or mandolin, slice potatoes very thinly.Place about 1 tablespoon of extra virgin olive oil on the bottom of a 9-inch oven-to-table dish and spread evenly. Sprinkle a few pinches of coarse salt and crushed red pepper flakes, if using, over oil; this will allow you to season both the top and bottom of the potatoes.Arrange your potato slices vertically in the dish, so the edges, not the flat sides, are up.Thinly slice shallots with a sharp knife or a mandolin. Place shallot slivers between potato slices, distributing them as evenly as possible. Brush with remaining 5 tablespoons extra virgin olive oil. Generously season with salt; lightly season with the crushed red pepper flakes, if using.Bake 1 1/4 hours, then arrange thyme sprigs on top and bake until potatoes are cooked through with a crisped top, about 35 minutes more. If potatoes seem to brown too fast, cover with foil. Serve immediately.